The use of liquid crystals (LC) for the creation of electrically tunable-focusing LC lenses (TLCLs), has been explored since few decades [S. Sato, “Applications of Liquid Crystals to Variable-Focusing Lenses,” Optical Review, Vol. 6, No. 6 (1999) 471-485]. One of the earliest geometries proposed was the TLCL based on the non-planar LC cell gap [S. Sato, “Liquid-Crystal Lens-Cells with Variable Focal Length,” Japanese Journal of Applied Physics, V. 18, No. 9, pp. 1679-1684, 1979], such as the one shown in FIG. 1a (PRIOR ART). The TLCL cell (10) uses a non-planar liquid crystal cell gap (12) sandwiched between two transparent glass substrates (14) covered by planar transparent electrodes (15), and transversely confined between rigid spacers (16). The cell gap separating the substrates is of a thickness d and includes a non-planar layer (13) filled with LC (17). In the example shown, inside the LC cell gap there is an element (18) made of transparent optical material such as an optical adhesive and characterized by a plano-concave profile and a refractive index nm which matches the ordinary index of refraction no of the LC (defined by the orientation of its director n that is the average direction of long molecular axes). The value of effective refractive index neff may be changed with respect to no=nm by applying an electric voltage V across the LC cell gap, up to an extra-ordinary value ne. The relative contrast of refractive index neff(V)−nm may thus be changed resulting in a change of focal distance F(V) of the whole system. Assuming the following typical parameter values: h=15 μm; r=0.8 mm→θ≈h/r=15/800 rad≈1.07 deg. Thus, in that typical example, the optical power may be estimated as follows: OP=2(ne−no)×h/r2=2×0.2×15×10−6 m÷0.64÷10−6 m2. It can be appreciated that with such optical parameter values, that type of TLCL could be very useful for small aperture applications such as mobile phone or web cam autofocus.
Heretofore, the industrial fabrication of a LC cell layer with curved internal surfaces, which requires precise LC alignment and orientation amongst other technical requirements, has proved to be a very difficult task when employing the typical manufacturing method that uses mechanical rubbing, which will be now described in view of FIG. 1b (PRIOR ART). A flat glass substrate (14), of typically 0.5 to 0.75 mm thickness, is first coated by an optically transparent and electrically conductive material, such as, e.g., Indium Tin Oxide (ITO), to form a transparent electrode (15). Then the ITO electrode is coated by a thin layer of alignment material (20), such as SiOx or Polyimide. In the case of the SiOx, the deposition conditions (angle, gas atmosphere, temperature, etc.) are chosen in a way to obtain the desired alignment and pre-tilt angle. In the case of the polymer material, the alignment and pre-tilt directions are imposed by the rubbing in a predetermined direction (22) of the layer by a rotating cylinder-type device (24). The position of the rubbing device relative to the substrate is controlled to maintain the working surface of the device in close contact with the exposed surface of the alignment layer to ensure accurate alignment of the LC molecules with high anchoring energy and the predetermined pre-tilt angle when assembling two such substrates (14) as shown in FIG. 1c (PRIOR ART). A LC cell sandwich is assembled with appropriate deposited or rubbed directions and spacers (16) to build a planar LC layer (26) with the desired pre-tilt angle α exhibited by LC director n. The cell spacers are usually dispersed across the volume of the cell gap or may be mixed with an adhesive and printed at the cell periphery. In view of the above examples, it can be appreciated that the deposition angle and the controlled close contact of the cylinder's surface with the alignment layer are critical parameters. That is why the precise alignment of LC on curved internal surfaces, providing appropriate pre-tilt angle and anchoring strength, is not an easy task and therefore the main effort of research and engineering community has been until now focused on flat TLCL solutions.